Tap changing apparatus for transformers

ABSTRACT

Tap changing apparatus for a transformer includes an off-load tapping selector switch having its contacts connected to tappings on a coarse tapping winding, and a rotary diverter switch connected to the tapping selector switch and to tappings on a fine tapping winding. The arrangement enables any required part of the fine tapping winding to be connected in series with any required part of the coarse tapping winding.

United States Patent Shorr0ck 1 Sept. 19, 1972 TAP CHANGING APPARATUSFOR TRANSFORMERS [72] Inventor: John Douglas Shorrock, 49 CavendishRoad, Heaton Mersey, Stockport, England [22 Filed: April2, 1971 21Appl.No.: 130,711

[30] Foreign Application Priority Data Feb. 27, 1970 Great Britain..9,494/70 [52] US. Cl. ..323/43.5 R, 336/ 146 [51] Int. Cl. ..H02p13/06, H02m 5/12 [58] field of Search ..336/146; 323/435 R [5 6]References Cited UNITED STATES PATENTS 3,122,698 2/1964 Ladd ..323/43.5R

3,423,668 l/l969 Brennan ..323/43.5 R 3,174,097 3/1965 Bleibtreu..323/43.5 R 3,461,379 8/1969 Okita ..323/43.5 R

Primary Examiner-Gerald Goldberg Attorney-Cameron, Kerkam & Sutton [57]ABSTRACT Tap changing apparatus for a transformer includes an off-loadtapping selector switch having its contacts connected to tappings on acoarse tapping winding, and a rotary diverter switch connected to thetapping selector switch and to tappings on a fine tapping winding. Thearrangement enables any required part of the fine tapping winding to beconnected in series with any required part of the coarse tappingwinding.

4 Claims, 6 Drawing Figures TAP CHANGING APPARATUS FOR TRANSFORMERS Thisinvention relates to tap changing apparatus for transformers.

Tap changing apparatus is provided for power transformers to enable theoutput voltage of the transformer to be maintained substantiallyconstant under varying load conditions, or to enable the output voltageto be varied. The tappings may be on either the primary or the secondaryside of the transformer, and the tap changing apparatus is usuallydesigned so that the tappings may be changed without disconnecting theload.

On-load tap changing apparatus consists basically of tapping selectorswitches arranged to operate only under no-load conditions and arrangedto provide electrical connection to two adjacent tappings on thetransformer winding, and an on-load diverter switch operable to selectone or the other of the two positions of the selector switch. Changingthe tapping on such an arrangement is thus a case of operating thediverter switch and the selector switches alternately, only moving theselector switch contacts which are not carrying current. In such a basicarrangement the number of output steps is equal to the number oftappings on the transformer winding. One of the problems arising fromthe use of tap changing apparatus of this type is that for each outputstep a separate lead has to be brought out from the transfonner winding.This may present a considerable problem when a large number of steps,possibly up to 30, is provided.

Tap changing apparatus is also known in which two selector switches areprovided connected to tappings on coarse and fine tapping windingsrespectively, the selector switches still operating only off-load. Theonload diverter switch is still required, and hence three switches arenecessary. This arrangement reduces the number of tapping leads butrequires the use of an additional switch.

It is an object of the invention to provide transformer tap changingapparatus capable of providing the required number of output stepswhilst requiring a considerably reduced number of tappings on thetransformer and using only two switches.

According to the present invention, there is provided tap changingapparatus for use with a transformer having electrically-isolated coarseand fine tapping windings in which the number of turns between adjacenttappings on the coarse tapping winding is n times the number of turnsbetween adjacent tappings on the fine tapping winding and in which thefine tapping winding comprises (n-l) sections, n being a positiveinteger greater than 2, the apparatus including an onload rotarydiverter switch arranged to be connected to the tappings on the finetapping winding and an offload tapping selector switch arranged to beconnected to the tappings on the coarse tapping winding, the tappings onthe fine tapping winding being so connected to the tapping selectorswitch and the rotary diverter switch that a selected part of the finetapping winding is connectable in series with a selected part of thecoarse tapping winding.

In this specification the term tapping is taken to include, whereappropriate, an end of the winding in question.

An embodiment of the invention will now be described, by way of example,with reference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a transformer connected to tap changingapparatus embodying the invention; and

FIGS. 2 to 6 illustrate the operation of the tap changing apparatus ofFIG. 1.

Referring now to FIG. 1, a transformer primary winding has a finetapping winding 10 and a coarse tapping winding 11. The coarse tappingwinding has a number of equally spaced tapping points, and the number ofturns of the winding between adjacent tappings is n times the number ofturns between the adjacent tappings on the fine tapping winding 10. Thefine tapping winding comprises (n-l sections such that the voltageacross the entire winding is equal to the voltage across one section ofthe coarse tapping winding. The four tapping points on the fine tappingwinding are denoted a, b, c and d, a and d being the two ends of thewinding. The coarse tapping winding 11 has one end connected to aterminal 12 and has five tapping points e, f, g, h and i, tapping ebeing the end of the winding opposite to that connected to the terminal12.

The tap changing apparatus, shown to the right of the chain-dotted linein FIG. 1, includes an off-load tapping selector switch 13 and anon-load rotary diverter switch 14. The tapping selector switch has tworows of fixed contacts 15 and 16, corresponding contacts in each rowbeing connected together as shown. These pairs of fixed contacts areeach connected to a separate one of the tappings on the coarse tappingwinding 11, and the fixed contacts are denoted by the reference 15 or 16as appropriate, together with a letter denoting the tapping to which thecontact is connected. The tapping selector switch 13 also has twomovable contacts 17 and 18. One of these, 17, makes electricalconnection with one of the fixed contacts 15, whilst the other movablecontact makes a connection with one of the fixed contacts 16. The twomovable contacts are provided with current collectors 19, and one ofthese, shown as 19d, is connected to the end d of the fine tappingwinding 10.

The rotary diverter switch 14 has four fixed contacts 20 arrangedsymmetrically around a circular path, and a movable contact 21 pivotedat the center of the path. One of the fixed contacts 20w is connected tothe current collector 19w of movable contact 18 of the tapping selectorswitch, whilst the other three fixed contacts 20 are connected to thethree remaining tappings a, b and c on the fine tapping winding, and aregiven appropriate references on the drawing. The movable contact 21 isconnected to a terminal 22. The movable contact 21 in fact has threeseparate contact surfaces. One of these is the main contact 23 which isconnected directly to the terminal 22. The other two contact surfacesare auxiliary contacts 24 and 25 located one on each side of the maincontact, and each is connected to the main contact through a separatediverter resistance 26 and 27 respectively. The angular spacing betweenthe three contact surfaces, and the size of each fixed contact 20 issuch as to give the diverter switch 14 the usual make-before-breakaction required for on-load switching.

It will be noted that only nine tapping connections have been broughtout of the windings. As will be described later, this enables a verymuch larger number of tapping steps to be obtained.

As is well-known in the art, it is necessary to provide an operatingmechanism and synchronizing means for ensuring the correct sequence ofoperations for the two movable selector contacts 17 and 18 and themovable diverter contact 21. However, such arrangements do not form partof the present invention.

The transformer will have a secondary winding which has not been shown.

The operation of he tap changing apparatus will now be described withreference to FIGS. 2 to 6.

If the two windings and 11 are considered as primary windings, then thesupply is connected between terminals 12 and 22. The connection, asshown also in FIG. 2, is thus from terminal 12 through the coarsetapping winding 11 to tapping g, through fixed contact 15g and movablecontact 17 of the tapping selector switch 13 onto the current collector19d, through the entire fine tapping winding 10 from tapping d totapping a, via fixed contact 20a of the rotary diverter switch 14 to themovable contact 21, and hence to terminal 22.

In order to increase the voltage from the secondary winding (not shown)or to compensate for a fall in voltage due to an increased load, it isnecessary to reduce the number of effective turns on the primary of thetransformer. This is done by rotating the movable contact of thediverter switch to fixed contact 20b, when the current path is as shownin FIG. 3.

As the movable contact 21 of the diverter switch is rotated in aclockwise direction the trailing" auxiliary contact makes contact withthe fixed contact segment 20a and the main contact 23 leaves the samefixed contact. The tapping a on the fine tapping winding 10 is thusconnected to the terminal 22 by way of the diverter resistance 27.Further movement of the movable contact causes the leading auxiliarycontact 24 to make contact with the fixed contact segment 20b, and hencethat part of the fine tapping winding between tappings a and b isbridged by the two diverter resistances 26 and 27 in series, thejunction between the two resistances being connected to terminal 22.Continued movement of the movable contact brings the main contact 23onto the contact segment 20b and the transition is complete.

A further reduction in the number of turns on the transformer primary iseffected by moving the rotary diverter switch further in a clockwisedirection, until the main contact 23 rests on auxiliary contact 20c. Thecurrent path through the primary is as shown in FIG. 4.

The next step of the diverter switch causes the lower end of the finetapping winding 10 to be disconnected from the current path by theengagement of contact 23 with auxiliary contact 20w, the moving contact18 of the selector switch now being on fixed contact 16g and carryingcurrent to the current collector 19d of the selector switch, as shown inFIG. 5. The upper end d of the winding 10 is now connected to fixedcontact 15g through collector 19d and contact 17 to prevent the windingfrom floating.

If the number of turns on the transformer primary is to be reduced stillfurther, then the movable selector contact 17 will have to be moved fromfixed contact 15g to fixed contact 15h whilst the other movable selectorcontact 18 is carrying the current. Further clockwise rotation of thediverter switch then causes the full fine tapping winding 10 to bebrought back into circuit in series with the selector contact 17 andcurrent collector 19d.

To increase the number of turns on the primary winding of thetransformer, the rotary diverter switch 14 is rotated in the opposite,anticlockwise, direction. The first step therefore cuts out the finetapping winding 10 and connects terminal 22 to tapping f on the coarsetapping winding, via the fixed contact 20w of the diverter switch andmovable contact 18 and fixed contact 16f on the tapping selector switch.The current path is then as shown in FIG. 6, the step describedfollowing that shown in FIG. 2. As before, the upper end of winding 10is connected to a tapping on the coarse winding 1 l to prevent it fromfloating.

Further anticlockwise rotation of the diverter switch reconnects thefine tapping winding via segment 20c of the diverter switch.

With the tap-changing arrangement described above, a total of 20 tappingsteps are available, although only nine tapping connections have beenbrought out of the windings.

As already stated, means are known for synchronizing the movements ofthe three movable contacts 17, 18 and 21 so that the selector switchcontacts move only when-they are not carrying current. As is usual, thediverter switch has a very rapid action so as to reduce the change-overtime to a minimum.

The tap-changing apparatus may be applied to the secondary of thetransformer if required, both fine and coarse windings then beingsecondary windings.

The rotary diverter switch has been described as having four positions,but this number may be varied, so long as it is greater than two. Sincethe object of the invention is to reduce the number of tappings on thewindings whilst providing the required number of tapping steps, theoptimum number of positions on the rotary diverter switch depends uponthe required number of steps. However, four appears to be a convenientnumber.

What I claim is:

1. Tap changing apparatus for use with a transformer having electricallyisolated coarse and fine tapping windings in which the number of turnsbetween adjacent tappings on the coarse tapping winding is n times thenumber of turns between adjacent tappings on the fine tapping windingand in which the fine tapping winding comprises (n-l) sections, n beinga positive integer greater than two, the apparatus including an onloadrotary diverter switch having n fixed contacts (n-l of which arearranged to be connected to successive tappings on the coarse tappingwinding and a movable contact capable of rotation in either directionand arranged to be connected to an output terminal, and an off-loadtapping selector switch arranged to be connected to the tappings on thecoarse tapping winding, the tappings on the fine tapping winding beingso connected to the tapping selector switch and the rotary diverterswitch that a selected part of the fine tapping winding is connectablein series with a selected part of the coarse tapping winding.

2. Tap changing apparatus as claimed in claim 1 in which the movablecontact of the rotary diverter switch comprises a main contact and apair of auxiliary contacts located one on each side of the main contactand each connected thereto through a separate diverter resister.

3. Tap changing apparatus as claimed in claim 1 in which the tappingselector switch has a plurality of fixed contacts arranged inelectrically-interconnected pairs with each pair arranged to beconnected to a separate tapping on the coarse tapping winding, each l0

1. Tap changing apparatus for use with a transformer having electricallyisolated coarse and fine tapping windings in which the number of turnsbetween adjacent tappings on the coarse tapping winding is n times thenumber of turns between adjacent tappings on the fine tapping windingand in which the fine tapping winding comprises (n-1) sections, n beinga positive integer greater than two, the apparatus including an on-loadrotary diverter switch having n fixed contacts (n-1) of which arearranged to be connected to successive tappings on the coarse tappingwinding and a movable contact capable of rotation in either directionand arranged to be connected to an output terminal, and an off-loadtapping selector switch arranged to be connected to the tappings on thecoarse tapping winding, the tappings on the fine tapping winding beingso connected to the tapping selector switch and the rotary diverterswitch that a selected part of the fine tapping winding is connectablein series with a selected part of the coarse tapping winding.
 2. Tapchanging apparatus as claimed in claim 1 in which the movable contact ofthe rotary diverter switch comprises a main contact and a pair ofauxiliary contacts located one on each side of the main contact and eachconnected thereto through a separate diverter resistor.
 3. Tap changingapparatus as claimed in claim 1 in which the tapping selector switch hasa plurality of fixed contacts arranged in electrically-interconnectedpairs with each pair arranged to be connected to a separate tapping onthe coarse tapping winding, each contact of a pair being engageable by adifferent one of two movable contacts.
 4. Tap changing apparatus asclaimed in claim 3 in which one movable contact of the tapping selectorswitch is connected to the remaining fixed contact on the rotarydiverter switch and the other movable contact of the tapping selectorswitch is arranged to be connected to one end of the fine tappingwinding.